The ingress of water into photovoltaic modules is known to cause performance loss related to corrosion, mechanical stress and optical transmittance degradation of encapsulating polymer. It is estimated that 2% of photovoltaic modules fail within 8 years of operation and that 16% of these failures are related to encapsulant degradation. Due to the long lifetime of modules (30 years) this problem is difficult to avoid even by using low water diffusivity materials. Mitigation approaches include the use of moisture barriers (glass) for the front and back sheet of the module as well as sealing the edges with desiccant filled polymers.
To evaluate durability and resistance to water ingress, conventional techniques involve incubating the photovoltaic module in hot, damp conditions for an extended period, such as 85 centigrade and 85% humidity for a period of 1000 hours.
After incubation, measurement of water content can be accomplished using one of two conventional techniques. First, measurement may be accomplished destructively by taking samples from different sections of the module. Second, measurement may be accomplished by using colorimetric reporters such as calcium films that exhibit a change in optical properties upon interaction with water.
Destructive evaluation has the downside that it is time consuming and measurement on the same module cannot be repeated, limiting the amount of collected data. Colorimetric tests, while non-destructive, can undesirably alter diffusion kinetics, may present additional issues related to integration with the fabrication processing, and cannot be performed on existing deployed modules unless those modules already contain appropriate reporter compounds.
There is therefore a need for fast, large-scale and stand-off evaluation method of water ingress that can reliably predict failure in photovoltaic modules.